Long story short, I filled the AX15 in my Jeep Cherokee with Mobil 1 75W-90 GL-5 gear oil about a year ago. After a few months, I noticed it was shifting a bit rougher. It was never great, and based on what I saw come out of the drain hole, I don't think the original owner ever changed the transmission fluid in its first 155K miles. Didn't think anything of it until I came across some comments on this here inter-web saying never to use a GL-5 fluid in the '96-'99 AX15's, because it eats the brass synchros. So, about a month ago, I drained the M1, and filled it with some Coastal GL-3,4,5 80W-90 fluid. Not surprisingly, its shifting even worse with this fluid. Then I read that GL-3,4,5 fluid really is just GL-5 fluid, and that, in fact, this stuff is even worse than the Mobil fluid I was using. Ugh. Today, I bit the bullet and ordered 4 quarts of Redline MT-90, for almost $50 after shipping. It best shift like butter when I put this stuff in there. The question is, should I drain the Coastal and maybe run some 10W-40 engine oil in there for a little while to clean out whatever sulfur was in the old stuff before I fill it with the MT-90? I'd like to get at least 20K out of this $50 worth of fluid, so I'm thinking maybe a "flush" with some nice, sulfur-free, engine oil isn't a bad idea.